The present invention relates to ink jet printing and, more particularly, to an ink jet printer in which printer operation and reliability at start up and shut down are enhanced.
Ink jet printers accomplish printing by depositing drops of ink on a print receiving medium in a pattern such that a print image is collectively formed by the drops. Typically, an ink jet printer includes a print head which defines a fluid reservoir in which electrically conductive ink is supplied. A plurality of orifices, arranged in one or more rows, are defined by an orifice plate mounted on the print head and each of the orifices communicates with the fluid reservoir. Ink is forced under pressure through the orifices and emerges as a plurality of fluid filaments. Varicosities are generated in the fluid filaments by mechanical stimulation of the orifice plate or by generating pressure waves which travel through the ink in the fluid reservoir. Fluid filaments are therefore caused to break up into streams of ink drops of substantially uniform size and spacing.
Charge electrodes are positioned beneath the orifice plate and adjacent the tips of the fluid filaments. Electrical charge potentials, selectively applied to the charge electrodes, induce corresponding charges of opposite polarity on the drops as they are formed from the filament tips. The drops then pass downwardly through a deflection field, with the charged drops being deflected by the field and the uncharged drops passing through the field in nondeflected trajectories. The amount of drop deflection is dependent upon a number of factors, including the level of charge carried by the drops. Some ink jet printers have operated in a binary fashion with the drops from each jet drop stream being either caught or deposited at a single print position. Such a printer is illustrated in Mathis U.S. Pat. No. 3,701,998. Other ink jet printers, such as for example that shown in Paranjpe U.S. Pat. No. 4,085,409, deflect the drops in each jet drop stream to a number of print positions.
At the start up of an ink jet printer, the fluid flow through the orifices and the formation of drops from the filaments are irregular and unpredictable. Exceptionally large drops of ink may be formed from the filaments and the trajectories of such drops are largely uncontrolled. As a consequence, there is a possibility that large amounts of ink may be deposited upon the charge electrodes and upon the deflection field electrode structure of the printer. If this occurs, the electrically conductive ink tends to short out the charge electrodes and the deflection electrode structure, and may also interfere with the trajectories of the jets once stable operation is obtained. Additionally, ink may be deposited on the print receiving medium transport and spoil the subsequently printed copies carried by the transport.
The large drops of ink which occur at start up cannot be predictably caught by a catcher in its normal operational position. Even with a catcher arrangement in which the catcher is positioned in line with the non-deflected trajectories of the jet drop streams and deflection of the drops is required for printing, the normal operating position of the catcher is one in which only a relatively small deflection of the drops is needed for the drops to clear the catcher and strike the print receiving medium.
Similar problems are encountered at shut down of the printer. As the pressure of the ink is reduced and fluid flow through the orifices is terminated, the jets once again become unstable and difficult to control.
Several approaches have been taken to overcome the problems presented by jet instability at start up and shut down. As shown in Van Breemen et al U.S. Pat. No. 4,081,804, a print head has been mounted over a drip pan at start up to collect drops formed from the fluid filaments until after the jets become stable. A print receiving medium is then transported beneath the print head and above the drip pan, and printing is initiated. The Van Breemen et al patent also discloses pivotal mounting arrangements for a pair of catchers in which the catchers can be pivoted downward and outward from the print head to permit inspection of the charge electrode structure.
A notched charge electrode plate is shown in IBM Technical Disclosure Bulletin, Vol. 20, No. 1, June 1977, pp. 33 and 34, which may be pivoted into an operating position after start up to reduce wetting of the charge electrodes. In an alternative arrangement, the charge electrode plate may be translated into its operating position. Pivoting of the charge electrode plate requires a substantial clearance in the printer structure. The translational mechanism dislosed is one in which the charge electrode plate is mounted on a spring arm and cammed out of its operating position. It will be appreciated that a spring mounting mechanism may be subject to undesirable vibration and, additionally, positioning of the charge electrode plate may be subject to dimensional inaccuracies.
IBM Technical Disclosure Bulletin, Vol. 19, No. 8, January 1977, pp. 3216 and 3217, discloses an ink jet printer in whih a pair of charge electrode plates are moved laterally into and out of operating positions after start up and prior to shut down, respectively. Additionally, a pair of catchers, positioned outwardly of the two parallel rows of jet drop streams during operation of the printer, are moved laterally together into contact at start up and shut down to prevent splattering of the ink on the print receiving medium. All of the drops are charged and deflected before the catchers are moved apart at start up and before the catchers are moved together at shut down. Since the catchers in the print head are moved together beneath the pair of rows of jet drop streams so that the streams strike the upper surfaces of the catchers, it is necessary that these upper surfaces be formed of a porous material to ingest the substantial flow of ink which they receive.
Keur U.S. Pat. No. 4,160,982 discloses an ink jet printing system having an accumulator or catcher which is positioned in line with the nondeflected jet drop stream during printing. Drops which are to be deposited on the print receiving medium are deflected away from the catcher. At start up of the printing system, the charging and deflecting electrodes are pivoted out of their normal operating positions and the catcher is raised such that it directly abuts the print head. After a stable jet drop stream is produced, the catcher is lowered and the charging and deflecting electrodes are pivoted into their normal operating positions. The pivoting mechanism for the charge and deflection electrodes requires a substantial clearance in the printing system. Additionally, the rack and pinion mechanism by which the catcher is raised is relatively bulky and accurate positioning of the catcher may be difficult.
An improved jet ink printing system is disclosed in Paranjpe et al U.S. Pat. No. 4,238,805. In the Paranjpe et al system, a print head is provided which generates two parallel rows of jet drop streams. A pair of charge electrode plates are movably mounted such that they may be translated into and out of drop charging positions. Each of a pair of catchers defines a drop catching surface and a drop ingesting slot along the lower edge of the drop catching surface. Each catcher is pivotally mounted for rotation about an axis parallel to the rows of jet drop streams. The catchers may be pivoted from drop catching positions, in which their drop catching surfaces are substantially parallel, to full catch positions in which their drop catching surfaces are inclined to face upward and intercept nondeflected jet drop streams. In the full catch position, the drop ingesting slots are positioned closely together.
A mechanical linkage system is provided in the device disclosed in the Paranjpe et al patent for pivoting the catchers from their full catch positions to their drop catch positions after start up of the printer. The linkage arrangement also moves the charge electrode plates into the drop charging positions. This occurs after the catchers are pivoted sufficiently to apply a drop deflecting potential thereto, but prior to rotation into their drop catching positions. While providing a substantial improvement in start up and shut down of an ink jet printer, the mechanical linkage arrangement for translating the charge electrode plates and rotating the catchers is relatively complicated. Additionally, since both the charge electrode plates and the catchers are actuated by a single linkage arrangement, the sequence and timing of movement of these printer elements may not be easily adjusted individually.
Accordingly, it is seen that there is a need for a simple, reliable, and compact ink jet printer in which start up and shut down of the printer are facilitated.